A load-carrying vehicle may be a truck or trailer unit, such as a dump or pickup truck. Such a vehicle may incorporate a tag axle at a rear end of the vehicle to increase a maximum payload rating of the truck or trailer, and to improve safety while driving the vehicle along a road surface. Some tag axles are retractable or otherwise liftable off the ground surface when not in use. Some tag axles incorporate shock absorbers. Some tag axles pivot when advancing through a valley or cresting a hill.